gpiod_get_value() gives out a warning if access to the underlying gpiochip
requires sleeping, which is common for I2C based chips:

    WARNING: CPU: 0 PID: 77 at drivers/gpio/gpiolib.c:2500 gpiod_get_value+0xd0/0x100
    Modules linked in:
    CPU: 0 PID: 77 Comm: kworker/0:2 Not tainted 4.14.0-rc3-00589-gf32897915d48-dirty #90
    Hardware name: Allwinner sun4i/sun5i Families
    Workqueue: events deferred_probe_work_func
    [<c010ec50>] (unwind_backtrace) from [<c010b784>] (show_stack+0x10/0x14)
    [<c010b784>] (show_stack) from [<c0797224>] (dump_stack+0x88/0x9c)
    [<c0797224>] (dump_stack) from [<c0125b08>] (__warn+0xe8/0x100)
    [<c0125b08>] (__warn) from [<c0125bd0>] (warn_slowpath_null+0x20/0x28)
    [<c0125bd0>] (warn_slowpath_null) from [<c037069c>] (gpiod_get_value+0xd0/0x100)
    [<c037069c>] (gpiod_get_value) from [<c03778d0>] (pwm_backlight_probe+0x238/0x508)
    [<c03778d0>] (pwm_backlight_probe) from [<c0411a2c>] (platform_drv_probe+0x50/0xac)
    [<c0411a2c>] (platform_drv_probe) from [<c0410224>] (driver_probe_device+0x238/0x2e8)
    [<c0410224>] (driver_probe_device) from [<c040e820>] (bus_for_each_drv+0x44/0x94)
    [<c040e820>] (bus_for_each_drv) from [<c040ff0c>] (__device_attach+0xb0/0x114)
    [<c040ff0c>] (__device_attach) from [<c040f4f8>] (bus_probe_device+0x84/0x8c)
    [<c040f4f8>] (bus_probe_device) from [<c040f944>] (deferred_probe_work_func+0x50/0x14c)
    [<c040f944>] (deferred_probe_work_func) from [<c013be84>] (process_one_work+0x1ec/0x414)
    [<c013be84>] (process_one_work) from [<c013ce5c>] (worker_thread+0x2b0/0x5a0)
    [<c013ce5c>] (worker_thread) from [<c0141908>] (kthread+0x14c/0x154)
    [<c0141908>] (kthread) from [<c0107ab0>] (ret_from_fork+0x14/0x24)

This was missed in commit 0c9501f8 ("backlight: pwm_bl: Handle gpio
that can sleep"). The code was then moved to a separate function in
commit 7613c922 ("backlight: pwm_bl: Move the checks for initial power
state to a separate function").

The only usage of gpiod_get_value() is during the probe stage, which is
safe to sleep in. Switch to gpiod_get_value_cansleep().

Fixes: 0c9501f8 ("backlight: pwm_bl: Handle gpio that can sleep")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>